Image orthicon

ABSTRACT

In the improved image orthicon the source of electrons required for the electron beam is obtained from a metal-oxide composite material, eliminating the thermal emitter and associated sources of heat and noise. The same metal-oxide composite material is used as the thin film target on which the image is formed, stored, and read. Image retention from preceeding frames is reduced to essentially zero and the orthicon structure is improved by elimination of the fragile thin glass targets in present use.

United States Patent 1191 Norman et al.

[111 3,866,078 [4 1 Feb. 11, 1975 IMAGE ORTHICON 221 Filed: Oct. 26, 1973 211 Appl. No.: 409,975

3,671,798 6/l972 Lees... 3l3/35l X Primary Examiner.lames B. Mullins Attorney, Agent, or FirmEdward J. Kelly; Herbert Berl; Jack W. Voigt [57] ABSTRACT In the improved image orthicon the source of electrons required for the electron beam is obtained from a metal-oxide composite material,-eliminating the thermal emitter and associated sources of heat and [52] U.S. Cl. 313/377, 313/391 [51] lnt. Cl. HOlj 31/26 nose The same metal'oxlde Composite mammal [58] Field of Search. 313/66 67 68 R 65 R used as the thin film target on which the image is gig T 5 formed, stored, and read. lmage retention from preceeding frames is reduced to essentially zero and the [56] References Cited orthicon structure is improved by elimination of the UNITED STATES PATENTS fragile thin glass targets in-present use.

3,067,348 12/1962 Ochs 313/67 5 Claims, 4 Drawing Figures L 48 i142 l0 12 l l .44 E} 54 i 52 J43 b I I h 5 B- 4o E L g: 38 "W IO\42 LQ OPT 36 14 ggg PATENTEB FEB] 1 I975 SHEET 2 [1F 2 co voo coo U000 coo coco coco Soc 000 photoemitting surface IMAGE ORTHICON DEDICATORY CLAUSE The invention described herein may be manufactured, used, and licensed by or for the Government for Governmental purposes without payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION The image orthicon is an image tube wherein a produces electrical images which are directed to one side of a target for storage. The other side of the target is scanned by an electron beam to obtain the output from the target. The basic image orthicon includes four functional sections: the image section, where light images are received; a target section where these images are stored as an electrical picture; the scanning stage where target readout occurs;.and the multiplier stage where the read out signal is amplified as a video output signal. Associated with all these sections are related acceleration grids, deflection coils, dynodes. and a thermionic electron source. The basic structure and operation of the orthicon is well established in the art and published in the literature and need not be disclosed in detail herein. However, a typical image orthicon and its simplified operation are set forth hereinbelow with reference to FIG. 1 of the drawings.

FIG. 1 discloses a simplified view ofa typical prior art image orthicon. An optical image is transmitted bya lens system and is formed on a photocathode 12. The light composing the image causes the photocathodic material to emit electrons which, by electrical biasing of a fine mesh screen 14, form an electron image on an ultra-thin sodium doped glass plate target 16 behind the mesh screen. In the center of base 18 of the image orthicon, a thermionic emitter (not shown) forms a cloud of electrons from which an electron beam is formed by an electronic lens. Typically, this device may be shown in the Radio Engineering Handbook by Keith Henney, McGraw-Hill Book Company, 1959. The beam is focused and caused to sweep the general area of glass plate 16 on which the image was constructed from the incoming optical signal.

Behavior of the electron beam must be fully understood to appreciate the operation of the device. An electron gun ZO directs the scanning electron beam with enough energy such that, if the potential of the target is zero, the scanning electrons will arrive at the target with zero energy. These electrons then return to the vicinity of the electron beam source and are multiplied by a secondary emission amplifier 22. If target 16 is positive at any point, some of the scanning electrons are extracted from the scanning beam to return that target area to a zero potential, and the remainder of the beam is returned to the multiplier 22. A signal is produced by this change in number of beam electrons returning to the multiplier.

The target 16 may comprise a thin glass sheet as previously stated or a thin film of magnesium oxide. The thinness required of these components results in their being very fragile and, thereby, makes the entire device fragile. The distance between the micromesh grid 14 and the target determines the storage characteristics of the tube and is the biggest single difference between orthicon tubes. The photocathode surface, on being struck by photons, generates photoelectrons which pass through this micromesh and strike the target. Secondary electrons are emitted by the target on impact and are collected by the micromesh. Emission of these electrons reduces the charge on the target at the point of emission, amplifying the signal by freeing more electrons than were gained by impact.

The orthicon heater for the emitter and related heat sinks add bulk and complexity to the existing systems. Power must be supplied to heat the emitter before electrons can be boiled off. This high heat, which is destructive to all circuit elements,'must then be dissipared. Gas evolved from the heater circuit is detrimental to tube operation. Thermal noise reduces efficiency. The tube requires a warm up time before the emitter will operate. Similarly, the glass plate target is fragile, making the tube fragile. The image storage density is limited by the doping density of the glass to provide sufficient conductivity therethrough while limiting charge migration toward the edges of the target. Additional structural and operational advantages and disadvantages of the orthicon are set forth in the literature. A comprehensive review of the image orthicon appears in Electro-Optical Photography at Low Illumination Levels by H. V. Soule, John Wiley & Sons, 1968.

SUMMARY OF THE INVENTION An improved image orthicon is disclosed wherein a new material, a metal-oxide composite, allows the orthicon to operate in an improved manner by reducing the fragile nature of the device, eliminating the thermal emitter and related thermal noise, and removing the heat generated from the area of the tube dynodes. The metal-oxide composite provides a source of electrons that is operable at ambient temperatures, eliminating the need for a thermal emitter and related heat sinks. The same composite provides a thin film target for the tube on which the image is formed, stored, and read. Image retention from preceeding frames is reduced to zero and the structural integrity is improved by the structurally sound composite as a target.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified diagrammatic drawing ofa prior art image orthicon, with typical support circuits omitted, such as accelerating grids and deflection coils.

FIG. 2 is a simplified diagrammatic drawing of the improved image orthicon employing the field effect emitter and charge carrier matrix, with extraneous structure omitted.

FIG. 3 is an enlarged, diagrammatic view of the electron emitter of FIG. 2.

FIG. 4 is an enlarged, diagrammatic view of the charge carrier matrix of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT The improved image orthicon employes a thin slice of a metal-oxide composite for storage of an image and from which the image may be read. Ultra-high packing density of metal fibers in the composite (matrix) allow accurate storage of electron images until the matrix is read. Insulation characteristics of the matrix oxide allow prolonged image retention by providing high impedance isolation of the charges stored on respective fibers.

The high density, metal-oxide matrix may be similar to that disclosed by Shelton et al (co-inventors in the present invention) in U.S. Pat. No. 3,745,402, issued July 10, I973. Shelton et al disclose that the metaloxide composite is a field effect electron emitter capable of emitting electrons at ambient temperatures when subjected to an electric field. The quantity of electrons emitted depend only on the electric field; thermionic emission is unnecessary. Also, a field effect electron gun'having at least a million fibers per square centimeter is disclosed in U.S. Pat. No. 3,783,325 issued Jan.

-I, 1974 to Shelton, a co-inventor in the present invention. Shelton discloses the electron gun can be shaped to produce a desirable electric field and current path for electron tubes. p i

The image orthicon of the instant invention may be best disclosed in reference to the drawings wherein like numbers represent like parts throughout the several drawings. FIG. 1 has already been disclosed in the Background of the Invention as being typical of prior art image orthicons. FIG. 2 discloses an improved image orthicon which utilizes the charge carrier matrix and field effect emitter, both being metal-oxide composites, while excluding extraneous portions of the tube. As may be seen in FIG. 2 an optical image is transmitted by lens and focused on photocathode 12. The light across the face of the image causes the photocathodic material to emit electrons which are di-- rected toward a target just as in the prior art devices. However, the target is a charge carrier matrix 36 hav ing a thinoxide coating 38 on one face thereof for transposing the impinging electron image into an electrical stored image. An electron gun assembly .40 having a field effectemitter 41 is disposed in the base of the orthicon with appropriately positioned control grids 42 for directing a scanning beam 44 toward the other face of target 36 for scanning the charge thereon. The returning beam 46 returns to an electron multiplier stage 48 surrounding emitter 41. The multiplier 48, which may be dynodes, is shown partially cutaway to disclose a. conductive plate 50 to which a direct current potential is coupled through a conductor 52. The conductor and conductive plate are shielded from the electron multiplier by appropriate insulating 54.

In FIG. 3, the electron gun assembly 40 is shown in greater detail. Field effect electron emitter 41 operates at ambient temperatures when placed in an appropriate electric field. The electric field is developed between the emitter and grids 42. Control grids 42 accelerate electrons from the emitter toward the target as scan beam 44. The emitter 41 may comprise from one to several million per square centimeter of metal emitting fibers 62 disposed in parallel and separated by an insulating matrix 63. Although metal fibers 62 may be pointed and protrude beyond the surface of insulator 63, they may alsobe left flush with the surface or recessed some degree by etching. Fibers 62 may terminate in a plane across the surface of the emitter as shown in FIG. 3.

In FIG. 4 charge carrier matrix 36 is shown to comprise a field effect emitting material wherein an oxide insulating matrix 66 supports metal fibers 67 and has thin oxide coating 38 on one face thereof. Oxide coating 38 functions as a secondary emitter of electrons and may be a thin film deposit of magnesium oxide, MgO, or of cesium antimony, CsSb, or other similar material. Typical of the insulating oxides, zirconium or uranium oxide may comprise elements 63 and 66, while tungsten may be the metalfibers 62 and 67.

Operation of the improved image orthicon is generally similar to that of the typical prior art orthicons. However, the charge carrier matrix 36 provides improved resolution that allows the outut video image to have high lateral resistance to charge migration per unit area and excellent fact-to-face conduction. This highly improves the image available under limited light conditions such as may be encountered withnight vision devices. The large number of parallel metal fibers and the oxide matrix insulator'are produced at high temperatures such that great structural strength results. Thus, the target 36 is structurally sound, eliminating a formerly inherent weakness and the resolution of the device' is greatly enhanced by the large quantity of image storage points available across the surface of the target.

Field effect emitter 40 allows elimination of th thermionic heater from the base of the orthicon. This substantially reduces thermal interference and interference from impurities and gaseous discharge generated by the prior art thermionic devices, allowing the dynodes or electron multiplier to operate with less noise interference.

When a photon image is focused through the glass envelope of the tube onto the photocathode 12, the photocathode gives off an electron image which is focused on the thin oxide coating 38. The oxide coating 30 emits electrons as secondary emission leaving a positive image on matrix 36. A beam of electrons from the field effect emitter is focused on the charge carrier matrix in a scan mode. This electron beam has energy per electron such that the electrons will stop short of the matrix if there is no charge on the matrix and will return to the electron multiplier for amplification as in the prior art. If the matrix is charged positively, the electron beam will neutralize this charge and return to the electron multiplier for amplification, modulated by the loss of electrons to the matrix.

While the invention has been described in connection with certain specific embodiments thereof, it should be understood that further modifications will suggest themselves to persons skilled in the art and it is intended to cover such modification as may fall within the scope of the claims appended hereto.

We claim:

1. In an image orthicon tube for converting impinging optical images into video signals, the improvement comprising: an electron gun for emitting an electron scan beam at ambient temperatures, said gun including a field effect electron emitter having several million parallel emitting fibers per square centimeter of emitting surface area, with the emitting ends of said fibers being terminated in a plane across the emitter surface, an insulating oxide encompassing and separating said fibers, and a conductive backing plate adjoining first ends of said fibers for applying an electric potential thereto; a field effect emitter charge carrier matrix target sheet disposed in spatial alignment with said gun, said target including a thin sheet of metal-oxide composite having several million parallel metal fibers per square centimeter of surface area and disposed substantially coaxial and parallel with said electron gun emitting fibers, said metal fibers being disposed normal to the surfaces of said target sheet, an oxide insulator between respective metal fibers for enhancing face-toface conduction through the sheet and retarding lateral conduction through the oxide, and a thin oxide coating on a first surface of said target sheet for providing secondary electron emission when stimulated by electron impact; and a photocathode disposed in alignment with said first surface of said target sheet. said photocathode emitting photoelectrons toward said target when stimulated by an input optical signal.

2. In an image orthicon tube as set forth in claim 1, the improvement further comprising: a fine mesh screen grid disposed adjacent to said first surface of said target for collecting secondary emission electrons emitted from said thin oxide coating; and first and second screen grids disposed adjacent said emitter surface between said emitter and said target for controlling the acceleration of said scan beam toward said target.

3. In an image orthicon tube as set forth in claim 2, the improvement further comprising an electron multiplier for amplifying the returning scan beam, said multiplier being disposed around the periphery of said emitter for capturing the returning electrons.

4. An image orthicon tube as set forth in claim 3, wherein said fiber emitting ends terminate in a plane forming the surface of said insulating oxide.

5. An image orthicon tube as set forth in claim 3, wherein said fiber emitting ends terminate in a'plane below the surface of said insulating oxide 

1. In an image orthicon tube for converting impinging optical images into video signals, the improvement comprising: an electron gun for emitting an electron scan beam at ambient temperatures, said gun including a field effect electron emitter having several million parallel emitting fibers per square centimeter of emitting surface area, with the emitting ends of said fibers being terminated in a plane across the emitter surface, an insulating oxide encompassing and separating said fibers, and a conductive backing plate adjoining first ends of said fibers for applying an electric potential thereto; a field effect emitter charge carrier matrix target sheet disposed in spatial alignment with said gun, said target including a thin sheet of metal-oxide composite having several million parallel metal fibers per square centimeter of surface area and disposed substantially coaxial and parallel with said electron gun emitting fibers, said metal fibers being disposed normal to the surfaces of said target sheet, an oxide insulator between respective metal fibers for enhancing face-to-face conduction through the sheet and retarding lateral conduction through the oxide, and a thin oxide coating on a first surface of said target sheet for providing secondary electron emission when stimulated by electron impact; and a photocathode disposed in alignment with said first surface of said target sheet, said photocathode emitting photoelectrons toward said target when stimulated by an input optical signal.
 2. In an image orthicon tube as set forth in claim 1, the improvement further comprising: a fine mesh screen grid disposed adjacent to said first surface of said target for collecting secondary emission electrons emitted from said thin oxide coating; and first and second screen grids disposed adjacent said emitter surface between said emitter and said target for controlling the acceleration of said scan beam toward said target.
 3. In an image orthicon tube as set forth in claim 2, the improvement further comprising an electron multiplier for amplifying the returning scan beam, said multiplier being disposed around the periphery of said emitter for capturing the returning electrons.
 4. An image orthicon tube as set forth in claim 3, wherein said fiber emitting ends terminate in a plane forming the surface of said insulating oxide.
 5. An image orthicon tube as set forth in claim 3, wherein said fiber emitting ends terminate in a plane below the surface of said insulating oxide. 